1. Field of the Invention
The present invention relates to a motor control apparatus for use in industrial applications and information devices.
2. Description of Related Art
FIG. 28 is a schematic diagram of a motor control apparatus according to the related art. This type of motor control apparatus is commonly used for controlling servo motors used in industrial applications. They are also used for motor control in information devices.
A typical motor control apparatus as shown in FIG. 28 is described below with reference to an industrial application using a three-phase motor 1.
In this typical application, a line current detector 6 detects the line current on two supply lines to three-phase motor 1, and determines the line current on the third line by taking the sum of the two detected line currents and inverting the sign. The results are output as first line current measurement iFU, second line current measurement iFV, and third line current measurement iFW. It will be obvious that this line current detector 6 could alternatively detect the line current on all three lines to the three-phase motor 1 to output first line current measurement iFU, second line current measurement iFV, and third line current measurement iFW.
A sensor 2 outputs signals required to detect the position, speed, and movement of a moving element of the three-phase motor 1. The sensor 2 is typically a rotary encoder or resolver.
A moving element position detection counter 7 outputs a moving element angle signal .theta., which expresses as an electrical angle the position of the moving element of the three-phase motor 1 based on the output from sensor 2.
A speed detector 8 detects the speed of the moving element of the three-phase motor 1 based on the output from sensor 2 and outputs a speed detection signal. A position detector 9 likewise detects the movement of the moving element of three-phase motor 1 based on the output from sensor 2, and outputs a position detection signal.
A position deviation signal indicative of the deviation between a position command used to drive the three-phase motor 1 to a specific moving element position, and the position detection signal output from the position detector 9, is applied to a position error amplifier 10. The position error amplifier 10 multiplies this position deviation signal by a position gain, and outputs the result as a speed command.
A speed deviation signal indicative of the deviation between the speed command output by the position error amplifier 10, and the speed detection signal output by the speed detector 8, is applied to a speed error amplifier 11. The speed error amplifier 11 similarly multiplies this speed deviation by a speed gain, and outputs the result as a current command.
The moving element angle signal .theta. output by the moving element position detection counter 7, and the current command output from the speed error amplifier 11, are applied to a line current command generator 12. The line current command generator 12 multiplies signal .theta. and the current command to output a first line current command iTU, a second line current command iTV, and a third line current command iTW. These line current commands indicate the line current to be supplied to the drive coil for each phase of the three-phase motor 1.
The first line current command iTU, second line current command iTV, and third line current command iTW output from the line current command generator 12, and the first line current measurement iFU, second line current measurement iFV, and third line current measurement iFW output by the line current detector 6, are input to an average current controller 13. The average current controller 13 generates switching command signals PU, PV, and PW so that the first line current command iTU and first line current measurement iFU, second line current command iTV and second line current measurement iFV, and third line current command iTW and third line current measurement iFW, are as close as possible to the same respective values.
A main circuit power element group 4 supplies power from a main circuit dc source 3 to the three-phase motor 1 based on the switching command signals PU, PV, and PW output by the average current controller 13.
A motor control apparatus according to the related art thus described operates as follows.
When the movement of the moving element has not reached the position indicated by the position command, the difference between the position command and the position detection signal, that is, the position deviation signal, is positive. The speed command output from the position error amplifier 10 therefore rises, causing the current command output from the speed error amplifier 11 to also rise. When the current command rises, the line current command generator 12 applies line current command signals to the average current controller 13 resulting in an increase in the line current supplied to each phase drive coil. The average current controller 13 supplies power from the main circuit dc source 3 to the three-phase motor 1 so that the line current flowing to the three-phase motor 1 conforms to the line current command signals.
When the movement of the moving element exceeds the position indicated by the position command, the inverse of the process described above is performed so that the movement of the moving element is again controlled to conform to the position command.
It will be obvious that by means of the operation described above, a motor control apparatus according to the related art is able to control a three-phase motor so that the position of a moving element thereof is controlled to a specified position.
As shown in FIG. 29, the above-noted average current controller 13 comprises three subtracters 41, 42, 43; first, second, and third current error amplifiers 44, 45, and 46; a triangular wave generator 50; and first, second, and third comparators 47, 48, and 49.
The subtracters 41, 42, 43 obtain first, second, and third line current deviations iEU, iEV, and iEW as the respective differences between first, second, and third line current commands iTU, iTV, and iTW and first, second, and third line current measurements iFU, iFV, and iFW.
The first, second, and third current error amplifiers 44, 45, and 46 generate and output voltage command signals VU, VV, and VW based on the first, second, and third line current deviations iEU, iEV, and iEW supplied thereto. The triangular wave generator 50 outputs a triangular wave signal SC, that is, a PWM carrier wave signal.
The voltage command signals VU, VV, and VW are input to the non-inverting input terminal, and the triangular wave signal SC is input to the inverting input terminal, of the first, second, and third comparators 47, 48, and 49, respectively. The comparators thus compare the supplied voltage command signals VU, VV, and VW and triangular wave signal SC, and output the result as switching command signals PU, PV, and PW, respectively.
Thus comprised, the average current controller 13 amplifies the difference between each line current command and line current measurement, represented by the respective current deviation, using a plurality of current error amplifiers, and then compare the resulting voltage commands with a PWM carrier wave signal to generate the switching command signals PU, PV, and PW.
The current error amplifiers 44, 45, and 46 used in this type of average current controller 13 are typically proportional integrating amplifiers with a gain characteristic as defined by equation (1). EQU G(S)=Kp(1+1/Ti.multidot.S) (1)
The control response of a control system using this type of proportional integrating amplifier can be improved by increasing the gain of the error amplifiers, and the standard deviation can be controlled to substantially zero by setting the integral. FIG. 30 is a waveform diagram of the internal operating signals of the above-noted average current controller 13 and the resulting line current signals flowing to the three-phase motor 1.
A problem with a motor control apparatus according to the related art as thus described is that an excessive increase in the current error amplifier gain causes oscillation due to the phase delay resulting from the electrical time constant of the three-phase motor and phase delay in the current error amplifier, as well as the operating delay of the power elements and excessive time delay generating a three-phase PWM signal. It is therefore common to set the gain of the current error amplifier as high as possible but below the level at which this oscillation occurs. This means that the total transfer function of the current control group must be considered during the design stage based on the characteristics of the three-phase motor, the motor current detector, current controller, and main circuit power group in order to determine the ideal gain setting. More specifically, this means that amplifier gain must be suppressed below the level at which oscillation occurs considering the worst-case scenario that could result from variations in these characteristics as a result of production differences and temperature characteristics.
For the above-described reasons, there is a limit to how high the current gain can be set in this type of conventional motor control apparatus. This further limits the improvement in response to the current command that can be achieved, and thus limits improvement in the response of a speed control and position control system external to the current control system.
Furthermore, while it is desirable to shorten the takt time of machine tools, robots, and other production line equipment in order to improve productivity in industrial applications, this requires improving the positioning response of servo motors used in these machines.
It is likewise desirable to improve the data access time of photocopiers, printers, hard disk drives, DVD drives, CD-ROM drives, and similar products in the information products industry. This also requires improving the control response of the motors used in such information products.
As described above, however, it is difficult to improve control system response with the above-noted conventional motor control apparatus because the current amplifier gain cannot be increased sufficiently. It is therefore not possible with a motor control apparatus according to the related art to achieve the required improvement in response as described above.